Seasonal redox dynamics and iron-controlled mobility of naturally occurring radionuclides in a tropical coastal aquifer

The presence of naturally occurring radionuclides in coastal groundwater constitutes a significant public health risk, especially in areas susceptible to hydrological variability. This research applies multivariate statistical methods, including principal component analysis and hierarchical cluster analysis, alongside the PHREEQC thermodynamic model to elucidate the mechanisms governing the mobility of naturally occurring radioactive materials (NORM), specifically gross alpha and beta activity, in a tropical coastal aquifer. Examination of 44 groundwater samples collected over two seasons indicated that more than half exceeded the national guideline for gross alpha activity (0.1 Bq/L). The results indicate that mildly acidic and reducing (anaerobic) conditions facilitate the reductive dissolution of iron and manganese hydroxides. The reductive dissolution of Fe/Mn oxides destabilizes the mineral structures that previously acted as sorptive barriers, thereby releasing trapped radionuclides directly into the groundwater. Additionally, this mobilization is synergistically influenced by competitive ion exchange mechanisms. PHREEQC modeling further identifies a potential iron "lock/release" mechanism: at mildly acidic conditions (mean pH 5.36-5.39), ferrihydrite precipitation is inhibited, maintaining elevated radionuclide mobility. Conversely, as pH increases (up to 7.32), the system approaches supersaturation (saturation index, SI > 0), promoting co-precipitation and resulting in a more than 31-fold decrease in alpha activity during the rainy season (from 0.405 to 0.013 Bq/L). In contrast, beta-emitting radionuclides are less affected by iron-related processes and are predominantly influenced by dilution, with a reduction of approximately threefold (from 0.358 to 0.109 Bq/L). These findings establish a scientific basis for developing cost-effective water treatment approaches, such as aeration combined with pH adjustment, to simultaneously remove heavy metals and alpha radioactivity from tropical coastal aquifer systems.